Refinancing a home loan changes the monthly payment and total interest owed, and a Mortgage Refinance Calculator shows that shift from the current loan to a proposed rate and term.
Calculate Mortgage Refinance Savings and Break-Even Timing with a Mortgage Refinance Calculator
A Mortgage Refinance Calculator compares an existing home loan against a proposed refinance offer, projecting the change in monthly payment, total interest, and the time needed to recover closing costs. It is used by homeowners evaluating a refinance quote from a mortgage lender before replacing their current loan.
Entering Current and Proposed Mortgage Details
Inputs include the current balance, monthly payment, and rate, plus the proposed loan amount, rate, term in years, closing costs, and whether costs are rolled in or paid upfront. Rates are entered as a nominal annual percentage compounded monthly. Outputs show the new payment, interest comparison, break-even point, timeline shift, and lifetime cost in the selected currency.
How the Refinance Payment, Interest Savings, and Timeline Are Calculated
The proposed monthly payment uses the standard amortizing loan payment formula, the same method reflected in mortgage amortization guidance published by the Consumer Financial Protection Bureau.
$$PMT = P \times \frac{r(1+r)^n}{(1+r)^n – 1}$$
P is the new loan balance — the proposed amount plus closing costs if they are rolled in, or the proposed amount alone if costs are paid upfront. r is the proposed rate divided by 12 and by 100. n is the proposed term in years multiplied by 12. When the proposed rate is entered as 0%, the payment is the balance divided by the number of months, since there is no interest to amortize.
A detail worth noting: the calculator never asks how many years remain on the current mortgage. Instead it solves for that figure directly from the entered balance, payment, and rate, using the standard loan-payoff formula:
$$n = \frac{\ln\left(\dfrac{P}{P – Br}\right)}{\ln(1+r)}$$
where B is the current balance, P is the current monthly payment, and r is the current monthly rate. This is the same math behind a lender’s amortization schedule, solved for time remaining instead of payment amount.
The break-even point — closing costs divided by the reduction in monthly payment — is a widely used convention in refinance planning for estimating how long it takes monthly savings to recover upfront costs. It is not a regulatory formula, and it assumes the payment difference between the two loans holds steady for the full term.
The calculator requires a current balance, current payment, and proposed loan amount above zero, a proposed term of at least one year, and interest rates of zero or higher. A current payment that does not exceed the monthly interest charge on the current balance is rejected, since the loan-payoff formula above has no defined solution for a loan that never amortizes down to zero. A proposed rate of 0% removes interest from the new-loan calculation, and no break-even point appears when the new payment is not lower than the current one.
Because the break-even estimate assumes the payment difference stays constant and does not account for changes in mortgage insurance, escrow, or a move before the break-even date, the output is a planning estimate rather than a lender-issued quote, tax determination, or personalized financial recommendation. The tool supports US Dollar, Indian Rupee, Euro, British Pound, Australian Dollar, and Canadian Dollar entries.
The amortization math is identical across currencies, but mortgage disclosure rules, permissible fees, and closing-cost norms vary by country and, within the US, by state, so figures should be checked against the lender’s Loan Estimate and Closing Disclosure.
Financing closing costs changes the total-cost picture in a way a simpler comparison would miss: the “Amortized Fee Cost” figure treats the rolled-in closing costs as their own small loan, carried at the new rate for the full new term, so a fee financed over a 30-year term can end up costing meaningfully more than its face amount by payoff — the calculator isolates that added interest rather than reporting the fee amount as-is.
Input Mistakes That Distort the Refinance Comparison
Entering a current monthly payment that does not cover the interest currently accruing produces an invalid result, since the calculator cannot back-solve a remaining term for a loan that would never pay down.
Entering the proposed term in months instead of years inflates the assumed loan length by a factor of twelve and understates the new payment. Selecting “Yes” to finance closing costs while expecting the same break-even timeline as paying upfront is also a common mistake, since financing the fee changes both the new balance and the new monthly payment.
Visualizing the Break-Even Point Between Closing Costs and Cumulative Savings
Common Questions About the Mortgage Refinance Calculator
Why doesn’t the calculator ask how many years are left on the current mortgage?
It calculates that figure instead of asking for it, using the current balance, payment, and rate to solve for the remaining number of months with the standard loan-payoff formula, the same approach behind a lender’s amortization schedule.
How does rolling closing costs into the new loan change the results?
The costs are added to the new loan balance and financed at the new rate for the full term, so the total cost increase from the fee is larger than the fee itself. Paying upfront avoids financing the fee but requires cash at closing.
Why is a current payment sometimes rejected as invalid?
If the payment does not exceed the interest accruing on the current balance that month, the loan would never amortize down to zero, and the loan-payoff formula used to derive the remaining term has no valid solution.
What happens if the proposed rate is entered as 0%?
The new payment is calculated as the new balance divided by the number of months in the term, with no interest applied, since the amortization formula has no interest component to compound at a 0% rate.
When does “No Break-Even” appear instead of a number of months?
It appears when the new monthly payment is not lower than the current one, since there are no monthly savings to recover the closing costs against.
Which currencies does the calculator support?
US Dollar, Indian Rupee, Euro, British Pound, Australian Dollar, and Canadian Dollar. The amortization formula works the same way in each currency; only the displayed symbol changes based on the selection.